1. Field of the Invention
This invention relates to a method and apparatus for drilling holes in mines for inserting roof bolts so that roof bolts can be inserted and fixed in roof rock faces to prevent their collapse.
2. Prior Art
Procedures utilized for the subterranean mining of coal have been greatly improved over the past several decades, both from the standpoint of operational safety on the part of miners as well as from the standpoint of their productivity. However, mining practices still are considered to be labor intensive, a factor significant in the pricing of coal. Additionally, current mining procedures necessarily continue to pose severe occupational safety difficulties. While current techniques of subterranean mining specific to a given strata being worked may represent a variety of technical approaches, the sequence of a given coal mining operation tends to follow a general pattern wherein machines of one variety or another work at the face of a seam to extract coal which then is conveyed outwardly from the mine. During this extraction procedure, there is created a progressively expanding subterranean cavern or chamber. As this procedure is carried out, the structural integrity of the immediately adjacent portions of the cavern roof or supporting portions is jeopardized. Consequently, the roof must be buttressed.
A variety of techniques have been developed and continue to be developed to achieve roof integrity; however, an important and most prevalent one of such techniques provides for the utilization of what are referred to in the art as “roof bolts”. A roof bolt assembly 8 is shown generally in FIG. 1. Typically, the procedure for bolting involves first, the carrying out of vertical and predetermined angular drilling through the roof of a recently mined area. This drilling normally will extend at least through a predetermined width of strata. Next, elongate steel bolts are inserted into the bores 6 and anchored therein, terminating at face plate 2 adjacent the cavern roof.
In the past, rotary drilling and coring tools, as used in mining and construction, have been constructed with hardened drill bit cutting heads, and traditionally with sintered carbide inserts to prolong the operative life of the tool. Typical cutting tools may use a single or continuous cutting surface or edge, but most frequently employ a plurality of discrete cutting elements or coring bits either sequentially or angularly arranged on a rotary bit or auger of some type.
A principal problem encountered in all of these prior art tools has been the rapid wear and high cost of replacement along with machine downtime. Such rapid tool wear and breakage, in part due to higher speed equipment and heavier frictional forces and tensile stress, has led toward tool redesign with some larger carbide insert or drilling tip configurations—which in some applications has resulted in higher dust levels and increased potential ignition dangers contrary to mining safety regulations. Pressurized water supplied to roof bit drilling operations adjacent to the drill bit has been employed to reduce dust and improve drilling rates.
Wet carbide drilling in the past utilized the delivery of water or other flushing fluids at low pressures in the range of 60-80 psi. The result of such prior art methods was that a single rotary drill bit using a sintered carbide insert, such as a roof drill bit of the type shown in the drawings, should be expected to drill at least one four (4′) foot bore before breaking or wearing out and might drill several of such bores, although in some hard rock formations, two or more prior art carbide bits might be required to drill a single 4′ bore. As detailed in U.S. Pat. No. 5,303,787, wet drilling increased performance and reduced dust and produced dramatic results even using the traditional methods of the prior art. Some comparison tests pertaining to water pressure changes only have been made in the industry; nine (9) insert rotary roof bits were operating at a conventional water pressure of 80 psi drilled 12,420 feet of rock for an average of 1,380 ft./bit. In this comparison test, eighteen rotary roof bits embodying the same configuration were operated in the same mine at water pressures of 300 psi and drilled 72,822 feet of rock for an average of 4,056 ft./bit.
In many instances, certain of the interconnected components of the drill steel are lost by virtue of their frictional engagement within the bore, which they have formed. For the most part, the drill steel components are interconnected by slideably mating male and female connections, which have no provision for providing tensional coupling to permit forced withdrawal from a bore. Some attempts to alleviate this drill steel loss have generally looked to the use of pins, which are driven through mating bores, which are formed within the female and male connections. However, such arrangements are found to be impractical in actual mining practice. The miner, generally operating in a posture somewhat near to prone, will remain entirely unappreciative of requirements for carrying punch and hammer first to insert, then to remove the pins as the drill steel is withdrawn from the bore. Such removal within a mine atmosphere is both hazardous and entirely impractical from a human engineering standpoint. Snap buttons have been adopted to simplify assembly of the drill steel and enable a miner to assemble the drill steel together quickly in a convenient manner. Such snap on coupling devices, however, are subject to leaking, resulting in undesirable water pressure losses in wet drilling operations.